1. Field of the Invention
This invention relates to a method for calibration of signal conditioning amplifiers and more particularly to a method of calibrating a signal conditioning amplifier which maintains data and integrity even in the presence of a hostile environment through the use of continuous calibration.
2. Prior Art
Across the electronics industry there are common problems that exist with field instrumentation. Signal conditioners frequently need to be removed from service for calibration. Calibration has traditionally been performed in a shop environment with traceable test equipment, typically on an annual basis. The need for shop calibration often keeps this particular maintenance from being performed routinely. Furthermore, this type of maintenance can usually only be accomplished during system outages or if a spare instrument can replace a device.
In cases where the instrument is invasive to a mechanical system, the removal of the instrument may require the use of labor from several different departments and trade group representatives. In order to avoid the need to remove components from field instrumentation for calibration, a need exists for a low-cost signal conditioner that is capable of performing continuous health checks and auto-calibration. This could significantly reduce the cost associated with periodic calibration and there would be no need to remove the signal conditioner from its operating location.
One attempt to calibrate amplifiers in the field has been to periodically interlace the signal from a transducer with a fixed calibration voltage reference. Unfortunately, this operation has typically resulted in a compromise between the amplifier bandwidth (also referred as analog bandwidth) and the acceptable noise level. The interlacing of the fixed reference voltage with the signal, sometimes at vastly different voltage levels, requires a large analog bandwidth to ensure that both, the signal from the transducer and the calibration reference voltage, could settle to acceptable limits (i.e., one part in 65536 for a 16 bit system). This requirement results in an analog bandwidth eleven times larger than the Nyquist frequency. This large analog bandwidth typically results in an aliasing problem as well as larger-than-desirable noise floor. U.S. Pat. No. 4,800,513 apparently utilizes this calibration technique.
U.S. Pat. No. 5,734,596 utilizes a signal and provides a first amplifier to assist in calibrating a second amplifier. Accordingly, the signal received by the first amplifier is not utilized to calibrate the first amplifier.
Accordingly, a need exists for a low-cost signal conditioner capable of performing continuous field checks and auto-calibration to significantly reduce operation and maintenance costs of a data acquisition system. Furthermore, a capability to perform continuous calibration will assure that short term variations and performance such as those caused by temperature changes, as well as long term variations such as those resulting from aging are compensated for in real time.
Consequently, an object of the present invention is to provide a low cost signal conditioner capable of performing continuous health checks and auto-calibration.
Another object of the present invention is to continuously calibrate an amplifier to correct for both short-term variations in performance as well as long-term variations in performance.
Yet another object of the present invention is to provide a calibration technique which does not result in a larger-than-desirable noise floor while still providing the desired analog bandwidth.
Accordingly, a transducer provides a signal to an amplifier such as through a low pass filter and a multiplexer. The output of the amplifier is preferably digitized and provided to a controller, such as a digital signal processor, which reads the voltage and estimates the voltage delivered by the transducer to the amplifier based upon the nominal gain of the amplifier. The estimated input voltage, locally generated, is then applied to the amplifier path immediately after the signal from the transducer has been digitized so that the calibration voltage is still close to the voltage of the time-varying sensor output. The output of the amplifier is then digitized again, and the values obtained from the application of calibration voltages as well as output voltages are compared to dynamically compensate for shifting gain and offset in the amplifier path. Accordingly, a smaller analog bandwidth is required and a reduced noise level is provided by this method and apparatus. Furthermore, reduced settling times for the output of the amplifier are provided since the calibration voltage is similar to the amplified voltage of the transducer signal. Although in the preferred embodiment presented here the digitized signal processing functions are implemented by the use of a Digital Signal Processor (DSP), these functions could be implemented with any controller technology such as but not limited to processors, microcontrollers, microprocessors, microcomputers or any other digital signal processing technology.